Seasonal affective disorder (SAD) is a subtype of recurrent depression involving major depressive episodes during the fall and/or winter months that remit in the spring. SAD affects an estimated 5% of the U. S. population, over 14.5 million Americans. The central public health challenge in the management of SAD is prevention of winter depression recurrence. The established and best available treatment, light therapy, remits acute symptoms in 53% of SAD cases. However, long-term compliance with clinical practice guidelines recommending daily use of a light box from onset of first symptom through spontaneous springtime remission during every fall/winter season is poor. Time-limited alternative treatments with durable effects are needed to prevent the annual recurrence of these disabling symptoms. Our preliminary studies suggest that a novel, SAD-tailored cognitive-behavioral therapy (CBT) may be as efficacious as light therapy alone for acute SAD treatment and that CBT may have superior outcomes to light therapy during the next winter. During the next wholly new winter season following the initial winter of study treatment, the proportion of depression recurrences was significantly smaller in participants randomized to CBT (5.8%) or to CBT combined with light therapy (5.2%) than in participants randomized to light therapy alone (39.2%). As the next step in this programmatic line of intervention studies, the primary aim of the proposed project is to further test the efficacy of our CBT for SAD intervention against light therapy in a larger, more definitive randomized head-to-head comparison on next winter outcomes in an intent-to-treat (ITT) analysis using all randomized participants. This project is seeking to test for a clinically meaningful difference between CBT and light therapy on depression recurrence in the next winter (the primary outcome), thereby having the potential to impact clinical practice. The proposed work will go beyond our pilot studies in four ways: (1) This study will augment the generalizability of our prior pilot study data by relaxing the inclusion/exclusion criteria to allow for comorbid diagnoses and by demonstrating the feasibility of training experienced community therapists to facilitate the CBT groups. (2) We will prospectively track recurrences and potential intervening variables that could affect outcome (e.g., new treatments, summer remission status) in the interim between treatment endpoint and the following winter. (3) This study includes a second annual winter follow-up to obtain preliminary data on the comparative effects of CBT vs. LT two winters after the initial winter of study treatment. (4) We will examine how potential modifiers influence the effects of CBT vs. LT, including demographic variables; baseline characteristics (e.g., depression severity, comorbidity); and complete or incomplete summer remission status in the interim. If successful, this work will develop a novel treatment with important public health implications for winter depression prevention.